I need help understanding the Magnetocaloric Effect. My understanding had been that based on the idea of conservation of entropy, if spin entropy decreases for the uncompensated electron spins then lattice entropy must increase so that entropy is conserved and so the magnetic material gets warmer.
My confusion is regarding magnetic materials with no or low Magnetocaloric Effect. If the material's uncompensated electron spins align so as to have lower entropy, then where does the entropy go so as to conserve entropy? If it went into the lattice vibrations then the material would get warmer and so the material would be classified as having a strong Magnetocaloric Effect.
If the entropy transfers to electronic states then there should be radiation from these changing electronic states but it ought to be higher frequency than the radiated heat emissions from lattice vibrations. The frequency would be lower for relatively massive nuclei vibrations as part of the lattice vibrating, so they would move relatively slower than the very low mass of electrons changing states. Electrons would have to move much more intensely in changing states to have the same heat energy of nuclei motions within lattice vibrations.
Besides spin states, electronic states and lattice vibrations, where else could the entropy disappear to?
Can there be a hidden increase in entropy among the compensated electron spins and among compensated proton and neutron spins, like a greater level of frustration or randomness among their unseen interactions with each other, I mean even while they are still considered as compensating each other?
If you're talking about an adiabatic process, any change in spin isotropy will inevitably lead to a change in the thermodynamic entropy, and thus, temperature. However, the extent of this change depends on many internal parameters, including the type of exchange coupling between the magnetic centers and their contributions to the existing phonon vibrations. In a system with a strong exchange interaction and large spin anisotropy, a large amount of thermodynamic work is needed to be done by the external magnetic field to align the spins. If this is done adiabatically, such a compressive force will cause an increase in the temperature, followed by a temprature decrease as soon as it is lifted.
This is, of course, not the case always. In some systems, spins are very localized and weakly coupled to one another. This accordingly makes them extremely susceptible to the external field, and hence, thermodynamically inactive. Furthermore, the localized electronic states of such magnetic centers are usually far from the Fermi level and therefore effectively absent in low-energy thermal excitations triggered by phonon vibrations, further reducing their magnetocaloric effect.
So in short, in a magnetic system, an adiabatic change in the spin distribution can lead to a magnetocaloric effect. To what extent is a question with no simple answer, at least not without an in-depth knowledge of the internal details of the system in question.
Thank Mohammad's comments that help me to understand the effect, as well.
My background is more in engineering than solid state physics. Please say if my understanding is correct. If an external electromagnetic signal from far away stimulates the spins of a magnetic material to move from an isotropic state to being very aligned then if all uncompensated spins change more or less coherently then they should radiate their own electromagnetic signal more or less coherently also? So spin entropy decreases and a signal is radiated that is capable of performing work exterior to the magnetic material?
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